The 8 3/4 in your son's '67 Dart has several things going for it.
1) the '67 is one of the lightest A-bodies
2) the tires he wants to use are going to be a limiting factor in terms of bite and therefore stress
3) this is supposed to be a "street car" and banzai launches should not be every launch particularly if he wants to keep his license
4) since he is using a stick, even if he breaks the rear he won't have 727 parts coming through the floor from a rolled sprag


That being said, I am a real beleiver in the brute strength of the Dana 60 when needed. I have never raced with anything else and never will